A little over a decade ago astronomers discovered something astonishing. They were looking at exploding stars in the distant universe, and they noticed that not only is the universe expanding from its big bang origin, but its expansion is actually speeding up.

Scientists call the culprit behind this acceleration dark energy. It's a mysterious entity that fills the universe and pushes things apart. And there seems to be plenty of it: recent estimates say that the universe is more than 70 percent dark energy. That's a pretty extraordinary claim, and it demands extraordinary evidence.

The WiggleZ team looked at how readily galaxies clump together, how much space separates those galaxies, and how things have changed over the past seven billion years. They found that something is indeed driving things apart. And dark energy fits the bill nicely. So dark energy has passed an important reality check. But what is it? On that front, scientists remain in the dark.

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